1. Field of the Invention
The present invention relates to soldering paste used to bond metallic materials and a soldering method using the same.
2. Description of Related Art
In order to package a part on the surface of a printed substrate, there is generally used a reflow soldering method, in which soldering is performed after following each of the processes shown in the process drawing of FIG. 2: soldering paste is bonded to a portion of a circuit pattern of the printed substrate by a printing method 1, a part is temporarily fixed to the printed substrate by means of adhesion, etc. 2, it is preliminarily heated at a temperature of about 150.degree. C. for several minutes 3, and thereafter, the solder paste is melted and heated at an elevated temperature higher than the melt temperature of soldering particles of soldering paste, for example, 230.degree. C. for scores of seconds 4, and is cooled 5 after the solder is melted for soldering.
The soldering paste comprises a mixture obtained by dispersing soldering particles in a flux mixture in which flux, solvent, adhesive mass and the like are mixed, and is printed or bonded to a printed substrate or the like by a screen printing method, etc.
The aforesaid printed soldering paste has paste-like viscosity at normal temperature, has an external shape having the same shape as the screen opening, and maintains a height equal to the thickness of the screen. However, it easily gets fluidized at high temperatures, and mostly starts being fluidized at a temperature of 50.degree. C. to about 150.degree. C.
When the soldering paste gets fluidized, its soldering particles flow into contact with the adjacent terminal electrode, causing a so-called defective bridge. The occurrence of this bridge causes a significant trouble in making the circuit pattern pitch narrower (0.5 mm or less) and miniaturizing the printed substrate.
FIG. 3 schematically shows the fluidization of soldering paste, or a process in which the defective bridge is caused; FIG. 3A shows a state before heating, and FIG. 3B shows the course of the reflow soldering process.
In FIG. 3, reference numeral 1 designates a part body such as QFP (Quad Flat Package); 2, a printed substrate; 3, a pad portion of a circuit pattern adapted to receive soldering paste; 4, lead wire for the part 1; and 5, soldering paste printed on the pad.
Reference numeral 6 designates soldering balls, which are soldering particles separated and isolated from a mountain of soldering paste 5 printed. They adhere to the substrate on printing, or are flowed and appear during heating, and these soldering balls 6 are fluidized in the heating process, thus causing a defective bridge.
In the reflow soldering process, as described above, a printed substrate 2, on which soldering paste 5 has been printed, is preliminarily heated as a preprocess for soldering by melting the solder.
This preliminary heating temperature is about 150.degree. C., lower than a temperature at which solder melts, but the viscosity of flux lowers during the preliminary heating, the neighborhood of the surface of the soldering paste 5 printed is fluidized so that the shape at the time of the printing is collapsed, thus causing sags 5a in the soldering paste 5.
The sags 5a flow into contact with the adjacent terminal electrode side when the solder is melted, causing a defective bridge. The defective bridge due to the sags 5a occurs more noticeably, the narrower the pitch becomes.
As a soldering method to solve the above-described problem, there is proposed a soldering method for soldering a circuit wiring and lead wire by bringing each joint of the lead wire into tight contact with the circuit wiring through cream solder, enveloping the each joint and the cream solder in ultraviolet-curing resin, and exposing the ultraviolet-curing resin to light to cure it for heating (Japanese Patent Laid-Open No. Hei 7-176856).
In the aforesaid soldering method, however, an extra process for enveloping in the ultraviolet-curing resin is compelled to be added in, for example, the reflow soldering process.
It is an object of the present invention to provide soldering paste capable of soldering without causing any defective bridge based on sags in the soldering paste occurring during the aforesaid preliminary heating, and without adding any extra process, and a soldering method using the soldering paste.